1. Field of the Invention
This invention relates to a method and apparatus for detecting and identifying a submerged object in a medium containing particulate matter causing poor visibility. More specifically, this invention relates to a high speed imaging method for detecting and identifying a submerged object in a murky or turbid medium.
2. Description of the Related Art
Generally, when light is directed at an object, the reflection of the light allows one to see the object. Suspended particulate matter, such as dust, mud (in water) and water droplets (fog), however, scatter both direct and reflected light greatly reducing visibility. If it reaches the observer, light which has been forward or backscattered distorts the image of the object. Only light that has not been either forward or backscattered and which reaches the observer forms an accurate image of the object.
One solution to eliminating the effect of forward scattered and back scattered light has been to gate an optical imager so that only a minimum of scattered light is seen. Unscattered light travels the shortest path from its source to the object and then to the observer. Thus, if one gates an optical imager to open at the precise point in time when unscattered light will arrive at the optical imager and to close after the unscattered light has arrived at the imager, then most scattered light will be prevented from obscuring the image of the object. In effect, by properly gating the optical imager, a clearer image of the object is formed. This use of a gated optical detector has been discussed in several U.S. patents. For example, see U.S. Pat. Nos. 3,682,553; 3,902,803; 4,862,257; 4,920,412; 4,967,270; and 5,013,917. Additionally, in U.S. Pat. No. 3,151,268, gate widths or exposure times of 3 nanoseconds or less are used. In U.S. Pat. No. 3,467,773, gate widths or exposure times of 20 nanoseconds are used. In U.S. Pat. No. 3,499,110, gate widths or exposure times of 10 nanoseconds are used. U.S. Pat. No. 3,527,881 suggests that the problems associated with underwater exploration are due to "backscatter," "forward scatter," and "attenuation." See column 2, lines 27, 44 and 51, respectively. U.S. Pat. No. 3,856,988 provides for an imaging system for environments where light scattering reduces image quality.
Turbid water inherently backscatters light. In underwater exploration systems, the intrinsic absorption and scattering properties of the water limit the usefulness of prior art systems. In addition to using short exposures or gate widths, the use of short pulsed sources of light also reduces the amount of scattered light entering a gated optical imager while it is gated open. In water, the source of illumination is generally a green or bluegreen laser. If the illuminating source is "on" continuously, there is a constant amount of scattered light present in the medium. In such an environment, where there is an abundance of scattered light present, the use of a gated optical imager GOI is defeated because the amount of scattered light entering the GOI cannot be greatly reduced. The effect of using a continuous source of light rather than a pulsed source is to introduce into the system an increased level of background noise or background scatter that will necessarily enter the GOI whenever it is gated open irrespective of the duration of the gate width. This scattered light, or backscatter, reduces the contrast between objects in the scene. Thus, underwater imaging systems most often operate in a pulse gated mode. This method of operation has been referred to as a range-gating technique.
It is suggested that the shorter the pulse width of the light source and the shorter the gate width of the gated optical detector, the better the clarity of the image obtained using a range-gating technique. However, given a short pulse width and a short gate width, little else has been done to improve the image quality of photographs of an object submerged in a turbid medium such as water. There is a need for a method for improving the image quality of an object in a turbid medium other than by just reducing the pulse width and the gate width.